1. Field of the Invention
The present invention relates to a heat-insulated four-cycle engine with a prechamber, the engine having a combustion chamber which is heat-insulated from the exterior of the engine and also having intake ports and a discharge port which are isolated from each other.
2. Description of the Prior Art
Conventional engines produce mechanical energy, i.e., an engine output, which is about 1/3 of the heat energy generated by combustion of fuel in combustion chambers. The rest of the generated heat energy, i.e., about 2/3 of the heat energy, is either transferred to the exterior through the engine itself or discharged out of the engine together with the exhaust gases. This heat energy is not added to the engine output, but simply wasted.
Various heat-insulated engines have been proposed to increase the engine efficiency. In the proposed engines, the inner wall surface of a combustion chamber is covered with a heat insulating material such as ceramic which has a low heat conductivity and high heat resistance, so that the combustion chamber is heat-insulated from the exterior of the engine. With this arrangement, the amount of heat energy discharged with the exhaust gases is increased, and the increased heat energy of the exhaust gases is recovered by a turbocharger or the like, for thereby effectively utilizing the energy of the exhaust gases.
The temperature in the combustion chambers of the heat-insulated engines is higher than the temperature in the combustion chambers of ordinary engines. Therefore, unless the air-fuel ratio were varied, the amount of nitrogen oxides (NOx) contained as an air pollutant in the exhaust gases would be increased. If the amount of fuel to be supplied to the engine were increased for the reduction of the NOx, then hydrocarbons (HC), carbon monoxide (CO), and black smoke would be discharged in increased quantities.
Since the exhaust gases are of a high temperature, the region around the exhaust port is heated to a high temperature. If the intake and exhaust ports are located closely to each other, the heat energy from the region around the exhaust port is transferred to the region around the intake port, resulting in a rise in the temperature of the region around the intake port. Intake air which is supplied through the intake port is then heated and expanded before it is introduced into the cylinder, and hence the actual mass of the drawn air is reduced. As a result, the engine output is lowered.
Another problem is that the wall surface of the combustion chamber is heated to a high temperature, tending to ignite the fuel earlier than the normal ignition timing, so that the engine suffers knocking.